Alternating current generator for vehicle

ABSTRACT

An alternating current generator for vehicles in which the exciting current flowing in the field winding (2) is detected and the current thus detected is suppressed not to exceed the maximum exciting current under the hot condition by the field current limiting circuit (5) for aiming at eliminating the generation of the maximum output under the cold condition to reduce the size of a pulley to result in a reduced vehicle weight and preventing the variation of the maximum output characteristic under the hot condition.

FIELD OF THE INVENTION

This invention relates to an alternating current (A.C.) generator to beinstalled on automobiles and the like.

DESCRIPTION OF THE PRIOR ART

FIG. 1 shows a circuit arrangement of a conventional A.C. generator, inwhich the reference numeral (1) denotes a three-phase output winding,(2) is a field winding mounted on the rotor of the generator, and (3) isa rectifying circuit for converting an A.C. output of the three-phaseoutput winding (1) into a D.C. (direct current) voltage output, which isused for charging a battery and at the same time, supplied to anelectric load. The reference numeral (31) denotes an auxiliaryrectifying circuit for outputting of the A.C. output as an excitingcurrent, and (4) is a voltage regulating circuit for regulating theexciting current for the field winding (2) to control the output voltageconstant, which is composed of a power tranistor (41) to make and breakthe exciting current, a driver transistor (42), a zener diode (43) and avoltage detecting circuit (44).

In the A.C. generator thus constructed, when the field winding (2) issupplied with current with the generator rotatably driven, the maximumexciting current will be determined by the resistance value of the fieldwinding (2), which causes the maximum output from the rectifyingcircuit. When the maximum output is not required, the voltage of theauxiliary rectifying circuit (31) rises to cause the potential of thedetecting circuit (44) to increase, which then makes zener diode (43)conductive to turn off the power transistor (41).

With the exciting current for the field winding (2) reduces, thepotential of the detecting circuit (44) is reduced to make the zenerdiode (43) non-conductive, which then causes the power transistor (41)to conduct. The repetition of the above mentioned operation will keepoutput voltage constant.

The output voltage of a generator of this kind has depended upon themaximum exciting current which is determined by the resistance of thefield winding (2) as shown above, and the maximum exciting currentvaries as this resistance varies to result in changes in the maximumoutput. Out of these changes is related to the temperature of the fieldwinding (2) to produce a cold output and a hot output. That is, if thecharging generator starts running from its cold, shutdown condition whenthe temperature of the field winding (2) is equal to that of ambientair, an exciting current in response to the resistance of the fieldwinding at that time will flow to produce the maximum output dependingon this current. This is what is called the cold output. As thegenerator keeps running for a period of time, the resistance of thefield winding (2) becomes increased by the heat generated in the fieldwinding (2) by the exciting current passing through that resistance. Theincrease in resistance then causes the exciting current to decreaseuntil heat generated and heat dissipated becomes in balance to produce aconstant output. This is what is called the hot output. Generallyspeaking, it takes several minutes to several tens of minutes for theoutput to shift from cold to hot. As the torque required to drive thegenerator is proportional to this output, the design condition should bedetermined depending upon the cold output when the torque is appliedthrough a pulley and the like which apt to slip against the appliedtorque and the size of a pulley to be used must be made large in size.FIG. 2 shows output characteristics of a generator of this kind, inwhich (401) is a cold output characteristic at an ambient temperature of20° C., (411) is a driving torque characteristic corresponding to thiscold output characteristic, (402) is a hot output characteristic at anambient temperature of 20° C., (412) is a driving torque characteristicunder the hot condition, (403) is a cold output characteristic at anambient temperature of 120° C., and (404) is a hot output characteristicat an ambient temperature of 120° C. The output characteristic (401) istransient in nature which is observed immediately after the starting ofvehicles and is not usually required for the same. However, to transmita torque having the driving torque characteristic (411) corresponding tothis cold output characteristic necessitates to use a pulley of 60 mm ofouter diameter for example. That is, a pulley large enough for the coldcondition has to be provided, which result in a large and heavygenerating system. Conversely, when an ambient temperature rises morethan a certain level, the design for the cold condition will cause aninsufficient output for the required output characteristic (405).

The other changes is caused by changes in the ambient temperature aroundthe generator. When atmospheric temperature is 20° C., the ambienttemperature around the engine of a vehicle will change in the range of20° C. to 120° C. The change in the ambient temperature varies theresistance value of the field winding (2), which causes the output tochange. This is so called the characteristic depending on ambiance ofthe output and the higher the ambient temperature, the greater theoutput decreases. Since a fixed amount of electric load is required fora vehicle under any ambient temperature, it is generally necessary toprovide an output with the ambient temperature rise taken intoconsideration.

The present invention therefore aims at eliminating the above mentionedproblems and providing an improved A.C. generator for vehicles in whichthe maximum output is keep constant without the cold output generatedtransiently and the output will not change by variation in the ambienttemperature.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved A.C.generator for vehicles provided with a field current limiting circuit tosuppress the maximum value of the exciting current fed to the fieldwinding so that the generation of the maximum output under the coldcondition is eliminated.

According to the present invention, no output of the generator under thecold condition will be greater than the output under the hot conditionso that a device such as a driving pulley specifically designed for thecold condition need not be provided. This will reduce the weight of avehicle on which the generator is to be installed and a stable generatoroutput which is not affected by the ambient temperature can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram showing a conventional A.C. generator forvehicles; FIG. 2 is a characteristic chart showing outputcharacteristics of the generator shown in FIG. 1; FIG. 3 is a circuitconfiguration diagram showing an embodiment of the A.C. generator forvehicles to the present invention; FIG. 4 is a characteristic chartshowing an output characteristic of the generator shown in FIG. 3; andFIG. 5 is a circuit configuration diagram showing an A.C. generator forvehicles another embodiment according to the present invention.

MOST PREFERRED EMBODIMENTS FOR ENFORCING INVENTION

The present invention will not be explained referring to theaccompanying drawings to make the same more precisely understood.

FIG. 3 shows an embodiment of the A.C. generator for vehicles accordingto the present invention, in which elements corresponding to those shownin FIG. 1 are denoted by the same reference numerals and the descriptionthereof is not repeated. The reference numeral (5) shows a field currentlimiting circuit which constitutes the main element of the presentinvention and is composed of the following components:

The reference numeral (51) denotes a current detecting resistor which isconnected between the emitter of a power transistor (41) and ground, andis provided to detect the exciting current flowing in a field winding(2); (52) shows a comparator which is composed to make a drivertransistor (42) conductive to cause the power transistor (41) to turnoff when current flowing in it exceeds a predetermined level; (53) and(54) are respectively a resistor and a zener diode for controlling theinput voltage to the comparator (52) constant; (55) and (56) areresistors for determining the reference voltage for the comparator (52);(57) and (58) are respectively a capacitor and a resistor connectedbetween the plus (+) input terminal and the output terminal of thecomparator (52) for suppressing oscillation thereof; (59) is a resistorconnected between the node of the resistor (53) and the resistor (55),and the output terminal of the comparator (52); and (50) is a diodeconnected between the output terminal of the comparator (52) and thenode of the base of the driver transistor (42) and the zener diode (43).

The maximum output under the hot condition (a predetermined value in therange of 20° C. to 120° C.) is guaranteed as a rated output in A.C.generator. The circuit according to the present invention makes avoltage developed across the resistor (51) when the maximum output isgenerated and feeds that voltage as the rated power for the comparator(52) by way of the resistors (55) and (56).

In the circuit thus configurated, when the electric load of athree-phase output winding (1) is so light that the field current valueis less than that at the maximum output under the hot condition, theoutput of the comparator (52) is at "low level", which will give noeffect on the field current control by a voltage regulating circuit (4).

On the other hand, under the cold condition, when the field currentvalue exceeds that at the maximum output under the hot condition withthe increased electric load, the output of the comparator becomes "highlevel", which makes a transistor (42) of the voltage regulating circuitconductive to cause a power transistor (41) to turn off so that thefield curent is interrupted.

By controlling the field current, which is the rated power output to thevalue at the maximum output under the hot condition with above mentionedoperation, the output characteristic and driving torque can be madeidentical as shown by the curve (406) in FIG. 4 so that the drivingtorque may be suppressed as in the case under the hot condition, ascompared with the case where the field limiting circuit (5) is notprovided (curve 406). Thus the load on the engine of a vehicle may bereduced.

In the embodiment thus constructed, since the maximum output of thegenerator under the cold condition can be made constant, the outerdiameter of a pulley driving the generator may be designed to be smallenough to prevent the slippage thereof. Generally speaking, the requiredouter diameter of the pulley is proportional to the square root of thegenerator output. If the constant output mentioned above is assumed tobe 60% of the cold output which is the transition output in theconventional generator, the actual size of the outer diameter of thepulley will be 46.5 mm, and the size of the pulley is about 80% whencompared with the conventional generator. All the other pulleys to becoupled with this pulley will also be reduced in size by the ratio of80% to contribute to the reduction of a vehicle weight. The generatoroutput thus not influenced by the ambient temperature makes an electricsystem of a vehicle stable and a highly reliable operation without aninsufficient output can be expected.

When manufacturers producing such A.C. generators for vehicles in massproduction perform their outgoing inspection, it is possible to measurethe hot output characteristic, which is the rated power output, one byone in the evaluation of the output characteristic of the generatorbecause of time limitations on their manufacturing operation. Therefore,it is a common practice to evaluate the generator by the cold outputcharacteristic which can be measured in a short period of time. However,in the case of the A.C. generator shown in FIG. 3, the evaluationthereof is not precisely performed since the field current limitingcircuit (5) functions to cause the output to become that of the hotcondition in the meansurement of the cold output characteristic.

The embodiment shown in FIG. 5 is intended to eliminate such a drawbackby selectively inhibiting the function of the field current limitingcircuit (5).

In FIG. 5, the reference numeral (6) denotes a battery and (7) is a keyswitch which connects the battery (6) (through a initial excitingresistor (8) with a voltage regulating circuit (4) and the outputterminal of an auxiliary rectifying circuit (31) to feed themoperational power. The reference numeral (9) is an electric load fedfrom the battery (6) and a rectifying circuit (3), and (10) is anindicating lamp which supplies the field winding (2) an exciting currentthrough the key switch (7) and a reverse-current stopping diode (11) forindicating by the lighting thereof that the generator is in anon-generating state. The reference numeral (12) denotes an externalswitch which composes the main element of this embodiment and isconnected to the node of the indicating lamp (10) and the power supplyterminal of the field current limiting circuit (5) at its one end, andto ground at the other end thereof.

In the circuit thus configurated, when the outgoing inspection of thegenerator is performed, the field current limiting circuit (5) can bemade inoperative by grounding the power supply terminal of the fieldcurrent limiting circuit (5) through a momentary closure of the externalswitch (12). Therefore, the output characteristic of the generator underthe cold condition (as shown by the curve (401) in FIG. 4) can beobtained as in the case where the field current limiting circuit (5) isnot provided. Thus an accurate and effective evaluation of the generatorunder cold condition can be performed. The lighting of the indicatinglamp (10) which occurs at the closure of the switch (12) will show aperson in concern that the field current limiting circuit is madeinoperative.

Although the above embodiment is configurated to ground the power supplyterminal of the field current limiting circuit (5) through the externalswitch (12), the switch (12) can also be inserted in series between thepower supply terminal and the indicating lamp (10) to effect the samecontrol operation.

In this case however, the lamp (10) dose not illuminate, so that therelease of the field current limiting circuit (5) will not be notified.The lamp (10) is shown to be connected between the key switch (7) andthe output termminal of the auxiliary rectifying circuit (31), but thislamp (10) can be also provided independent of the auxiliary rectifyingcircuit (31) to obtain the same effect.

In addition, although the above embodiment is described as to the effecton the outgoing inspection performed by generator manufacturers, it canbe utilized for the generator installed on a vehicle. That is, when theoutput of an A.C. generator on board is desired to be increasedtemporarily according to the running and electric load conditions of avehicle by making the field current limiting circuit (5) inoperative, acontrol switch provided on the vehicle corresponding to the externalswitch (12) in operation, which is actuated on and off according to saidrunning and electric load conditions of the vehicle can obtain the aim.

Thus the embodiment last mentioned can make the evaluation of the outputcharacteristic of the generator in the outgoing inspection accurate, andfurthermore, it is effective to release the function of the fieldcurrent limiting circuit (5) in the generator on board.

As described above, the present invention can provide an improved A.C.generator for vehicle of high practical current fed to a field windingto keep the predetermined maximum output characteristic under the hotcondition by a field current limiting circuit to result in a reducedvehicle weight and a highly reliable electric system for vehicle. TheA.C. generator according to the present invention is also capable ofmeasuring the output characteristic thereof effectively and accurately.

What is claimed is:
 1. An alternating current generator for vehiclescomprising a field winding, a three-phase output winding for generatingan output in response to the exciting current in said field winding, arectifying circuit for converting the three-phase output of saidthree-phase output winding into a direct current, a battery to becharged by the output of said rectifying circuit, a voltage regulatingcircuit for controlling the output voltage of said rectifying circuit toa predetermined value by intermittently controlling the exciting currentin said field winding, an indicating lamp connected between said voltageregulating circuit and said battery for indicating the generating ornon-generating states of said generator, a field current limitingcircuit fed with operating power through said indicating lamp forsuppressing the exciting current in said field winding to less than apredetermined value and an external switch connected to said fieldcurrent limiting circuit for selectively inhibiting the operation ofsaid field current limiting circuit.
 2. An alternating current generatoras set forth in claim 1, wherein one end of said external switch isconnected to the node of said indicating lamp and said field currentlimiting circuit and the other end thereof is connected ground.
 3. Analternating current generator as set forth in claim 1, wherein saidexternal switch is connected between said indicating lamp and said fieldcurrent limiting circuit in series.